chore: fix chain doc in toml (#341)

Author: ltitanb

config.example.toml

@@ -2,7 +2,7 @@
 # Some fields are optional and can be omitted, in which case the default value, if present, will be used.
 
 # Chain spec ID. Supported values:
-# A network ID. Supported values: Mainnet, Holesky, Sepolia, Helder, Hoodi.
+# A network ID. Supported values: Mainnet, Holesky, Sepolia, Helder, Hoodi. Lower case values e.g. "mainnet" are also accepted
 # A custom object, e.g., chain = { genesis_time_secs = 1695902400, path = "/path/to/spec.json" }, with a path to a chain spec file, either in .json format (e.g., as returned by the beacon endpoint /eth/v1/config/spec), or in .yml format (see examples in tests/data).
 # A custom object, e.g., chain = { genesis_time_secs = 1695902400, slot_time_secs = 12, genesis_fork_version = "0x01017000" }.
 chain = "Holesky"